Among the known active implantable medical devices are "double" or "dual" chamber devices which collect (sense) and deliver (stimulate) signals in the high cavity (the atrium) and the low cavity (the ventricle) of the heart. These devices are designed to follow the cardiac rhythm of the patient and to undertake some diagnosis and/or therapy functions of an atrial arrhythmia (AA) and/or a ventricular arrhythmia (AV).
A process for the control of such a double chamber cardiac pacemaker in the case of a detection of a ventricular extrasystole (ESV) is described, for example, in EP-A-0 550 342, and its corresponding U.S. Pat. No. 5,312,451, which are commonly assigned and which disclosure is hereby incorporated herein by reference. This document describes a means for preventing trouble of the cardiac rhythm that can occur at the appearance of a ventricular extra-systole (ESV). A ventricular extra-systole can induce a pause, also known as a ventricular pause, that favours or can precipitate the establishment of a trouble of the cardiac rhythm by the desynchronisation of ventricular refractory periods. In these conditions, a tachycardia can be established.
It is not possible to anticipate the occurrence of a ventricular extra-systole. However, it is possible, according to the process described in EP-A-0 550 342 and U.S. Pat. No. 5,312,451, to warn of the dangerous consequences of such an event.
According to this known process, at the detection of a ventricular extra-systole, one stimulates the atrium simultaneously with the appearance of the ventricular extra-systole, and then one stimulates the atrium at a rhythm that is more rapid than the rhythm existing before the appearance of the ventricular extra-systole, decreases the atrio-ventricular (AV) delay during this rapid stimulation, and returns the stimulation rate slowly by steps to the base frequency existing before the appearance of the ventricular extra-systole. The ventricular pause is thus avoided. This technique has been successfully implemented with the help of programs downloaded into a memory of a microprocessor-based active implantable medical device, especially in the double chamber devices such as the CHORUS brand pacemakers manufactured by the assignee hereof ELA Medical.
Nevertheless, some limitations to the foregoing control process have been observed in clinical practice.
One of the disadvantages of the process mentioned above is that it processes only extra-systoles of ventricular origin, and does not attend to atrial extra-systoles (ESA) giving rise to the same phenomenon of desynchronisation, which also is prejudicial to a good stability of the cardiac rhythm.
Further, it has been observed during an occurrence of the so-called "doublets" or "triplets" of extra-systoles, that are, respectively, a sequence or "salvo" of two extra-systoles (ESA or ESV) or a continuation of three extra-systoles without an intermediary event of non extra-systolic origin, the process is deactivated and specifically does not processed this type of cardiac event.
In addition, it has been noted that the variation of the rhythm during a phase of acceleration after the detection of a ventricular extra-systole was not sufficiently physiological, because it does not take into account the precocity of the extra-systole.
Also, during the repeated appearance of a ventricular extra-systole during the activation of the algorithm of the process, the former has a tendency to accelerate the rhythm to an increasingly high frequency, and typically to the programmed maximal frequency. Such a successive acceleration, that is normally favourable, presents the problem that very often the frequency of stimulation does not correspond to physiological needs of the patient. For example, an acceleration increasing the frequency from 100 to 120 bpm is not problematical, but an acceleration from 50 bpm to a maximal frequency (typically at 120 bpm) is not appropriate if the patient is at rest.